Pleasure boats can be grouped into at least three different categories by application and engine power. The lowest power boats are typically used for fishing and water skiing and have engines in the range of 25 to 150 horsepower. The middle category is generally classified as a sport boat and uses one or more engines in the 150 to 300 horsepower range. Boats using engines in excess of 300 horsepower are typically racing boats. Boats as defined above may exclude some cabin cruisers, yachts, and other ocean going vessels.
Boats which fall into the lowest and middle power ranges utilize engines which drive a propeller through a transmission as compared to the race boat category in which the engine is always driving the propeller through a direct drive. The boat transmission historically provides only for forward, reverse, and neutral and does not incorporate a clutch. Accordingly, shifting occurs under power, desirably at engine idle, and results in a significant shock load in the components of the drive system. On very low horsepower drives where the rotating propeller and hub assembly have relatively low mass, the shock load effect is usually acceptable. In this size category, the propeller hub assembly is usually pinned to the drive shaft with a "shear" pin. This pin can absorb the shock load of forward to reverse shifting but is designed to shear and allow the hub assembly to spin free if the propeller strikes an object, such as a log, in the water. The shear pin thus provides a degree of protection for the drive assembly, i.e., the engine transmission and drive shaft, of smaller horsepower systems.
As engines became larger, and hub assemblies more massive, it was necessary to develop an apparatus to absorb the greater shock loading during shifting and to replace the shear pin which could now be "sheared" by engine torque unless made so large that it became ineffective in preventing damage when an object was struck. The apparatus developed incorporated an elastomeric coupling between the propeller hub and the engine drive shaft. A drive member having a central aperture for fitting onto the drive shaft included an outer annular rubber element bonded to the drive member. The central aperture has a plurality of alternating grooves and ridges running axially to provide a non-slip connection via corresponding mating grooves and ridges on the drive shaft. The drive member is press-fit into an outer driven member to which propeller blades are attached. The coupling between the driven member and the drive member thus comprises the annular elastomeric element attached to the drive member. In practice, the press-fit may require as much as eight tons compression to compress the rubber element into the outer cylindrical driven hub member. The rubber element absorbs the shock loading during transmission shifting and also provides a break-away connection in the event that the propeller strikes an object.
The elastomeric coupling system has become an industry standard. Unfortunately, this system has been found to have disadvantages as horsepower available for pleasure boats has increased. When the elastomeric coupling was first developed, most pleasure boats used engines of less than 100 horsepower. When engine horsepower exceeds about 200 horsepower, it has been found that the impact of propeller blades on water at high torque levels generates sufficient force to cause slipping of the outer hub about the elastomeric coupling. Once any slipping has occurred, the efficacy of the coupling is severely downgraded. While power can still be generated in many cases by proceeding at slow speed, a slipped coupling requires replacement. Since this type of coupling appears to be limited to use below about 200 horsepower, it is desirable to provide a propeller hub assembly which can be used with higher horsepower sport boats.